An automobile has been required to be compacter and lighter, and to have an expanded passenger space, which needs the compacter engine room, and hence the compacter, lighter electronic device members and engine accessories. Moreover, the engine room is more closed to meet the requirements of noise suppression, which is accompanied by increased temperature within the engine room and hence needs the engine members to withstand higher temperature. These members are frequently arranged below the engine room, tending to be exposed to rainwater while running. Therefore, the grease for the rolling bearing in these sections is required to be more rust-preventive than those for other sections.
The bearing for, e.g., electrically driven fan motors has been used at 130 to 150° C. However, increased temperature in the engine room calls for the bearing which can withstand higher temperature of 180 to 200° C. One of the measures for the bearing operating at a conventional temperature level of 150° C. or lower is lubrication with the packed grease comprising a synthetic base oil incorporated with a urea-based compound, as disclosed by, e.g., Japanese Patent No. 1,982,669. This grease, however, causes seizure in an early stage when used at high temperature of 160° C. or higher. Therefore, more heat-resistant greases have been in demand.
It is known that the bearing is serviceable in an atmosphere of 160° C. or higher, when packed with, for example, a fluorine-based grease comprising a perfluoropolyether oil as a base oil which is incorporated with polytetrafluoroethylene (PTFE) as a thickening agent to improve resistance to heat. However, it tends to have deteriorated rust preventive performance, because of difficulty in incorporating a rust preventive agent to be used in the common synthetic oil-based grease. Dispersion of a solid rust preventive agent may be one method, which, however, greatly deteriorates the acoustic performance of the grease. The fluorine-based grease has another disadvantage of being 5 to 20 times more expensive than the synthetic oil-based grease.
The grease composition comprising a urea-based grease incorporated with fluorine oil for improved resistance to heat is known, as disclosed by Japanese Patent Laid-open Publication No. 11-181465. Such a grease composition, however, suffers the notable phase separation, because of no affinity of the base oil of mineral or synthetic oil for the fluorine oil, and hence unsuitable for the bearing which supports members rotating at a high speed, e.g., electrically driven fan motors. Another type of known grease comprises a base oil comprising hydrogenated mineral or synthetic oil and fluoropolyether oil, as disclosed by Japanese Patent Laid-open Publication No. 07-268370. However, this grease involves the problems of the phase separation and low stability to shear stress, because of low thickening agent content of 3 to 20% by mass based on the amount of the grease.
As described above, the bearing for electrically driven fan motors and couplings for cooling fans which send cooling air to automobile engine radiators, being set in an atmosphere of higher temperature, tends to have decreased service life before seizure. It is therefore necessary to further improve resistance to heat of the grease for the bearing, to secure the service life. At the same time, the grease for the bearing is required to be more rust-preventive than those for other sections, because the bearing is arranged in a section tending to be exposed to rainwater while running. Moreover, it is required to have sufficient fluidity at low temperature, to prevent noise when the engine is started under a low temperature condition. However, the bearing packed with the conventional grease cannot meet the requirements of the electrically driven fan motors and couplings.